An ultraviolet sensor has been used, for example, as a flame sensor for a fire alarm device or a combustion monitoring device of a burner and, in addition, has also been used as an ultraviolet detection device of a simple apparatus measuring the amount of ultraviolet radiation in an outdoor environment. Furthermore, an ultraviolet sensor has also been expected as a next-generation optical communication device which uses ultraviolet rays.
As the ultraviolet sensor, for example, a sensor using a diamond semiconductor or a SiC semiconductor have been developed and have been already produced on a commercial basis. However, in the case of the ultraviolet sensor using a diamond semiconductor or a SiC semiconductor as described above, there have been problems in that materials therefor cannot be easily machined and are also very expensive.
Hence, in recent years, research on ultraviolet sensor using an oxide semiconductor material has been pursued, and for example, a diode-type sensor having sensitivity in an ultraviolet region has been proposed, the structure of which has a hetero junction formed from an n-type semiconductor layer primarily composed of a titanium oxide and a thin film composed of a transition metal oxide (see, for example, Patent Document 1). In the ultraviolet sensor disclosed in this Patent Document 1, the n-type semiconductor layer primarily composed of titanium oxide is formed of a single crystalline substrate, and the transition metal oxide film is formed of a thin film obtained by epitaxial growth. In this case, the single crystalline substrate and the thin film described above are processed by a heat treatment, such as annealing, but these are not sintered bodies. In addition, in the ultraviolet sensor disclosed in the Patent Document 1, the transition metal oxide thin film is disposed at an ultraviolet ray receiving side.
However, the ultraviolet sensor disclosed in the Patent Document 1 has the following problems.
Since the n-type semiconductor layer primarily composed of a titanium oxide, which is used at a base side of the ultraviolet sensor, has high intrinsic resistance, the electrical conductivity is low, and hence the sensitivity of the ultraviolet sensor is relatively low. The properties described above are backed up, for example, by data shown in FIG. 5 of the Patent Document 1. In addition, since the transition metal oxide thin film is disposed at the light receiving side, the light transmission rate is also low. By the configuration described above, the sensitivity is also degraded.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-172166